Abstract
1. A study was made of nitrogen-fixation byAzotobacter chroococcumalone in a medium containing dextrose (which it can utilize) and in mixture with a coliform organism on a medium containing no carbohydrate except starch, whichAzotobactercannot utilize unless it be hydrolysed by the coliform organism or some other agency.2. The amount of nitrogen fixed in the mixed cultures was found to be maximal at two temperatures, and a discussion is given of the causes thought to be operative in producing the double maximum.

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